1. Field of the Invention
The present invention concerns in general terms a multi-user detection method (MUD). More precisely, the present invention relates to an iterative method of eliminating interference between users (Multiple Access Interference or MAI). The present invention applies more particularly to mobile telephony in DS-CDMA mode (Direct Sequence—Code Division Multiple Access) that is to say to mobile telephony using a code division access mode with spectral spreading by direct sequences.
2. Description of Related Art
In a DS-CDMA mobile telephony system, the different users are separated by multiplying each symbol of the user by a spreading sequence peculiar to it, also referred to for this reason as the signature of the user, the different signatures ideally being chosen so as to be orthogonal. The spreading sequence frequency (chip rate) being greater than the frequency of the symbols, the signal transmitted by each user is distributed (or spread) in the frequency space. On reception, the signal of a user is separated by virtue of a filtering adapted to the corresponding signature. This filtering is also referred to as “despreading”. The ratio between the band occupied by the spread signal and the band occupied by the information signal is referred to as the spread factor.
The signatures employed must have good correlation properties, namely a very pronounced auto-correlation peak and low intercorrelation values.
The first of these two characteristics allows synchronisation of the sequence received. It is very useful when the transmission channel of a user has several propagation paths. This is because each path must then be isolated by virtue of a filtering adapted to the signature and to the delay of the path. It is possible to take advantage of the diversity of propagation within the channel to increase the signal to noise ratio on reception. To do this, a bank of adapted filters, separating the different paths, is used and the outputs of them are combined. The most widespread combination is MRC (Maximum Ratio Combining), which consists of multiplying the signal output from each adapted filter by the conjugate of the complex multiplicative coefficient introduced by the channel on the path concerned. The resulting filtering operation is a filtering adapted to the equivalent filter of the channel. Through its structure, the receiver thus formed is referred to as a rake receiver. Naturally, perfect separation of the paths takes place only if the auto-correlation is a Dirac. In practice, however, separation is not complete and leaves multipath interference which is also referred to as self-noise. FIG. 1 depicts schematically a DS-CDMA system with K users. The data of a user k are spread in frequency by the corresponding signature in the module 100k before being transmitted over the channel 110k having P paths. On reception, for a given user k, the signals being propagated according to the different paths p=1 . . . P of the channel are separated by adapted filters 120k,1 . . . 120k,P (only the battery of filters of the user k has been shown) before being weighted by a set of complex coefficients ck,p. The signals thus weighted are summed (140k) and the resulting sum output from the rake receiver is subsequently detected to provide an estimation of the data of the user k. In the case of a downlink (links from a base station to a mobile terminal) the channels 1 to K are identical, whilst they are different in the uplink (links from mobile terminals to the base station). The first case can, from this point of view, be considered to be a particular case of the second.
The second characteristic set out above guarantees a low level of interference between two distinct users. Nevertheless, here also, in practice, the intercorrelation between two signatures is rarely zero. This is notably the case in a so-called dazzle situation (the Near-far effect) where a high-power signal received from a user interferes with the reception of a low-power signal coming from another user. Moreover, when the number of users is high, close to the spread factor, the sum of the interferences of the different users, low if taken in isolation, can have very disturbing effects on detection.
In order to combat the multi-user interference, several methods have been proposed. A review thereof will be found in the article by Simon Moshavi entitled “Multi-user detection for DS-CDMA communications”, which appeared in IEEE Communications Magazine, October 1996, pages 124–136. Amongst existing multi-user techniques, the techniques of subtractive elimination (Subtractive Interference Cancellation) have good performance with reasonable complexity in use. The general idea of it is simple; from a first detection at the output of the adapted filter, the contributions to the interference suffered by the other users are reconstructed by respreading. This interference is then subtracted from the signal received in order to provide a cleaned signal at a subsequent detection step. According to the way in which the subtraction is carried out, there may be parallel elimination (PIC, standing for Parallel Interference Cancellation) and serial elimination of interference (SIC, standing for Serial Interference Cancellation).
Let it be assumed first of all that the signal of each user propagates on a single path as far as the receiver.
The parallel elimination method is illustrated in FIG. 2: the received signal is filtered by a battery of adapted filters (2001,2002, . . . , 200K), each adapted filter relating to a given user. After detection (210k), the estimated symbols are respread (220k) spectrally by means of the signature of the user in question before being filtered by a filter modelling the transmission channel (230k). There is thus, at the output of (230k), an estimation on the contributory share of the signal received attributable to the user k. The sum of the contributory parts of the other users is then subtracted (at (240k)) in order to obtain a cleaned signal Sk(1). This cleaned signal can directly be the subject of detection after despreading or the elimination process can be reiterated. The detection being of better quality at each iteration, there is then obtained, at the end of successive iterations, signals Sk(i) which are more and more rid of the multi-user interference.
The serial elimination method is illustrated in FIG. 3: the signals received by the different users are first of all ordered in decreasing order of power, that is to say 1, . . . , K. The procedure then consists of successive eliminations of the contributory shares, commencing with the signal of highest power. To this end, the SIC detector has a series of stages in cascade, each eliminating the interference due to a particular user. The first stage works on the antenna signal and each subsequent stage receives as an input the output of the previous stage. Each stage has an adapted filter (300k), a detector (310k), a module (320k) for respreading the symbols, a filter (330k) modelling the transmission channel k and a subtractor (340k) eliminating the contribution due to the user k. Each stage also supplies at the output of the detector (310k) a decision on the symbol received, Ŝk, and the interference elimination process ends at the Kth stage.
The techniques set out above apply well to the simple situation where the transmission channel of a user has a single path. In this case, the filter modelling the channel can be limited to multiplication by a complex coefficient. When the channels are multipath, the situation is on the other hand much more complex since it is necessary to eliminate both the multipath interference and the multi-user interference. An iterative detector with subtractive elimination of the multi-user interference when there are multiple paths was proposed in an article by M. C. Reed et al. entitled “Iterative Multiuser detection using antenna arrays and FEC on multipath channels” published in the IEEE Journal on Selected Areas in Communications, Vol. 17, No 12, December 1999, pages 2082–2089. Each iteration of the detection comprises an adapted filtering, a channel formation and a combination of the rake type. The method proposed presupposes however that the attenuation coefficients, the phase rotations and the directions of arrival of all the paths of all the users are determined. This determination is carried out externally to the detector, prior to the sequence of iterations and therefore on signals interfered with by multi-user and multipath interference. As a result the elimination of this interference is necessarily approximate.